Integrated circuits often require supply voltages of greater potential than that provided by an external voltage source (external supply). Memory circuits such as dynamic random access memories (DRAMs) and video DRAMs require higher internal voltages to pre-charge memory word lines and the like. Flash memories may require high voltages for programming operations, such as erasing. Integrated circuits that are dependent upon a limited external power supply, such as a battery, must generate additional supply voltages using conversion circuitry. Charge pumps have been used as on-chip voltage generators capable of providing a voltage more positive than the most positive external supply or more negative than the most negative external supply.
In certain circumstances, the external supply may already be at a high voltage level already. One such circumstance includes life-cycle testing of an integrated circuit during the manufacturing process. Life-cycle testing includes a high-voltage, high temperature test to eliminate integrated circuits that have an undesired probability of failure within a predetermined amount of time. In this test, the charge pump may generate from the high-voltage external supply an even higher voltage level than intended. This higher voltage level may cause damage to an integrated circuit, that otherwise may not have the undesired probability of failure, which reduces the yield of saleable integrated circuits during the manufacturing process.
Several techniques have been introduced in an attempt to alleviate the problem, including the use of diode clamps and diode stacks. However, these techniques are engaged after the integrated circuit has built up a potentially damaging level of voltage, making the effectiveness of these techniques questionable. Additionally, these techniques cannot be turned on or off as needed.
Thus, what is needed are devices and methods to selectively control the generation of high voltages in charge pump circuits.